Improving the performance of Group III-nitride (e.g., AlGaInN) based devices, including light-emitting diodes (LEDs), has been the intense focus of research and development efforts worldwide. While current LEDs are based on planar thin-film architectures, vertically aligned nanowires (also called nanorods or nanocolumns) are being explored as an alternative architecture. Several advantages of nanowire-based devices and LEDs have been recently reported. For example, nanowire-based LEDs can enhance light extraction due to light scattering, optical mode elimination, and efficient light out-coupling. See C. H. Kuo et al., IEEE Photonics Technology Letters 22, 257 (2010). Strain-relaxed, bottom-up nanowire growth also enables high crystalline quality with significantly reduced threading dislocation densities. See S. D. Hersee et al., Nano Lett. 6, 1808 (2006). Higher indium compositions, which are desired for longer green-red wavelength emission, can be achieved with nanowires because of their compliance properties and strain relief mechanisms. See Q. M. Li and G. T. Wang, Appl. Phys. Lett. 97, 181107 (2010); and T. Kuykendall et al., Nature Mater. 6, 951 (2007). Variability in the emission wavelengths across nanowires within an ensemble can lead to phosphor-free “white” LEDs. See H. W. Lin et al., Appl. Phys. Lett. 97, 073101 (2010); and H. Sekiguchi, K. Kishino, and A. Kikuchi, Appl. Phys. Lett. 96, 231104 (2010). Additionally, suppressed quantum confined Stark effect (QCSE) and reduced droop in InGaN/GaN nanowires have also been reported. See C. Y. Wang et al., Opt. Express 16, 10549 (2008); and H. P. T. Nguyen et al., Nano Lett. 11, 1919 (2011). Finally, the geometry of nanowires allows them to act as lasing cavities, enabling nanosized lasers.
Growth of InGaN/GaN-based nanowire LEDs by bottom-up methods, including hydride vapor phase epitaxy and molecular beam epitaxy, have been demonstrated. See H.-M. Kim et al., Nano Lett. 4, 1059 (2004); H. W. Lin et al., Appl. Phys. Lett. 97, 073101 (2010); H. P. T. Nguyen et al., Nano Lett. 11, 1919 (2011); and K. Kishino et al., “InGaN/GaN nanocolumn LEDs emitting from blue to red—art. no. 64730T,” in Gallium Nitride Materials and Devices II, H. L. C. W. Morkoc, ed. (SPIE, Bellingham, Wash., 2007), pp. T4730-T4730. However, relatively low growth temperature and low V to III ratio are commonly used to promote anisotropic one-dimensional crystal growth. Metal catalyzed-grown nanowires also require narrow growth conditions which involves lower than optimal growth temperatures. See G. T. Wang et al., Nanotechnology 17, 5773 (2006). These growth conditions may introduce higher impurities and point defect densities than the conditions used for creating commercial-quality planar LEDs and provide less flexibility for adjusting growth parameters to optimize doping concentrations and other desired material properties. See A. A. Talin et al., Appl. Phys. Lett. 92, 093105 (2008); and P. C. Upadhya et al., Semiconductor Science and Technology 25, 024017 (2010).
In contrast, nanowires fabricated by top-down methods are etched from planar thin film structures grown under optimized growth conditions, obviating these disadvantages. Tapered, non-faceted InGaN-based nanowire LEDs have been previously demonstrated by plasma etching planar LED structures. See C. Y. Wang et al., Opt. Express 16, 10549 (2008); and C. H. Chiu et al., Nanotechnoloqv, 445201 (2007). However, the top-down plasma etching leads to damaged, rough, and non-faceted sidewalls with defects, and leakage currents that limits performance. See C. Y. Wang et al., Opt. Express 16, 10549 (2008).
Therefore, the present invention is directed to a top-down method for fabricating vertically aligned Group III-V micro- or nanowires using a two-step etch process that adds a selective anisotropic wet etch after an initial plasma etch to remove the dry etch damage while enabling micro- and nanowires with straight and smooth faceted sidewalls and controllable diameters independent of pitch.